System and method for rapid generation of low par Q-mode signals

CROSS-REFERENCE TO RELATED APPLICATIONS

N/A

INCORPORATION BY REFERENCE

N/A

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

BACKGROUND OF THE INVENTION

0004Current ADSL modem system designs do not incorporate a low power transmission mode. Such systems require high power dissipation in the modem line driver, even when no data is being transmitted.

0005Accordingly, ITU contributions have proposed a low power mode (i.e., “Q-mode,”) in the transmitter. During the proposed Q-mode, the modem is still in the ready state, but enters a low power mode during periods of no data transmission.

0006ITU contributions, HC-029R1 and AB-045, formally define a semi-stationary Q-mode signal, that employs a pair of pseudo-random bit sequence (“PRBS”) generators, each with a period of greater than 4000. These proposals require two separate signals, namely, a stationary signal and a non-stationary signal. One problem with the resulting Q-mode signal is that it does not have a sufficiently low Peak-to-Average Ratio (“PAR”). The “Average” used may be, for example, the root-mean-square. A signal having a low PAR would enable the analog front end of the modem to reduce the amount of power dissipated in the line driver even further during Q-mode. By reducing power dissipation, a Q-mode signal having a low PAR would correspondingly reduce the amount of heat generated in cabinets containing ADSL equipment. A reduction in heat would in turn make it possible for ADSL service providers to either reduce the size of cabinets for the same number of ADSL modems, or to deploy more ADSL modems per cabinet than is possible with current Q-mode proposals.

0007Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

0008Aspects of the present invention may be found in a communication system comprising a data communication node, such as an ADSL modem, for example. A transmitter is located in the data communication node, which uses a PRBS generator to generate multiple signals. The transmitter determines the Peak to Average (e.g., root-mean-square) (“PAR”) ratio of each of the signals generated, and outputs at least an indication of the signal having the lowest PAR. The signal having the lowest PAR, or at least an indication thereof, is used to generate a non-data mode signal. Such indication may be, for example, the state of the PRBS generator that generated the signal having the lowest PAR.

0009In an embodiment where the data communication node is an ADSL modem, the signals generated by the PRBS generator may be ADSL signals, and the non-data mode signal may be a Q-mode signal.

0010In one embodiment, the indication of the signal having the lowest PAR output by the transmitter is communicated to a remote receiver, and is used by the remote receiver to receive the non-data mode signal. Again, the non-data mode signal may be, for example, a Q-mode signal in an ADSL application.

0011These and other advantages and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram of a generic communication system that may be employed in connection with the present invention.

<figref idref="DRAWINGS">FIG. 2</figref> is a depiction of a generic PRBS generator in accordance with the present invention.

<figref idref="DRAWINGS">FIG. 3</figref> is one specific embodiment of the PRBS generator of <figref idref="DRAWINGS">FIG. 2</figref> in accordance with the present invention.

<figref idref="DRAWINGS">FIG. 4</figref> is a functional block diagram of one embodiment of a method for computing/determining the PAR of an ADSL symbol.

<figref idref="DRAWINGS">FIG. 5</figref> is a flow diagram of one embodiment for obtaining a signal with the lowest PAR.

<figref idref="DRAWINGS">FIG. 6</figref> is a more detailed flow diagram of one embodiment of a method for obtaining a signal with the lowest PAR.

<figref idref="DRAWINGS">FIG. 7</figref> is a flow diagram of one embodiment of communication between ADSL modems according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

0019<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram of a generic communication system that may be employed in connection with the present invention. The system comprises a first communication node <b>101</b>, a second communication node <b>111</b>, and a channel <b>109</b> that communicatively couples the nodes <b>101</b> and <b>111</b>. The communication nodes may be, for example, ADSL modems or any other type of transceiver device that transmits or receives data over a channel. The first communication node <b>101</b> comprises a transmitter <b>105</b>, a receiver <b>103</b> and a processor <b>106</b>. The processor <b>106</b> may comprise, for example, a microprocessor. The first communication node <b>101</b> is communicatively coupled to a user <b>100</b> (e.g., a computer) via communication link <b>110</b>, and to the channel <b>109</b> via communication links <b>107</b> and <b>108</b>.

0020Similarly, the second communication node <b>111</b> comprises a transmitter <b>115</b>, a receiver <b>114</b> and a processor <b>118</b>. The processor <b>118</b>, like processor <b>106</b>, may comprise, for example, a microprocessor. The second communication node <b>111</b> is likewise communicatively coupled to a user <b>120</b> (again a computer, for example) via communication link <b>121</b>, and to the channel <b>109</b> via communication links <b>112</b> and <b>113</b>.

0021During operation, the user <b>100</b> can communicate information to the user <b>120</b> using the first communication node <b>101</b>, the channel <b>109</b> and the second communication node <b>111</b>. Specifically, the user <b>100</b> communicates the information to the first communication node <b>101</b> via communication link <b>110</b>. The information is transformed in the transmitter <b>105</b> to match the restrictions imposed by the channel <b>109</b>. The transmitter <b>105</b> then communicates the information to the channel <b>109</b> via communication link <b>107</b>. The receiver <b>114</b> of the second communication node <b>111</b> next receives, via communication link <b>113</b>, the information from the channel <b>109</b> and transforms it into a form usable by the user <b>120</b>. Finally, the information is communicated from the second communication node <b>111</b> to the user <b>120</b> via the communication link <b>121</b>.

0022Communication of information from the user <b>120</b> to the user <b>100</b> may also be achieved in a similar manner. In either case, the information transmitted/received may also be processed using the processors <b>106</b>/<b>118</b>.

0023In the case when the communication nodes <b>101</b> and <b>111</b> of <figref idref="DRAWINGS">FIG. 1</figref> are ADSL modems, the transmitters <b>105</b> and <b>115</b> use one or more PRBS generators to generate various signals. <figref idref="DRAWINGS">FIG. 2</figref> is a depiction of a generic PRBS generator that may be employed in connection with the present invention. PRBS generator <b>201</b> outputs a pseudo random bit sequence with a period of 2^N−1, where each of i₁, i₂, . . . i_Ncomprises one of “0” or “1” and {i₁, i₂, . . . i_N} (not all equal to “0”) represents the initial state of the PRBS generator <b>201</b>. {c₁, c₂. . . c_N} represents the coefficients of a primitive polynomial.

0024One such signal generated by the transmitters <b>105</b> and <b>115</b> (of <figref idref="DRAWINGS">FIG. 1</figref>) in ADSL communication is a REVERB signal, as defined in ITU recommendation G.992.1. The REVERB signal is constructed in the frequency domain using all available carriers numbered from 0 to 255, although normally many carriers are missing and one is appropriated for use as a pilot tone used for timing control. Each carrier is modulated using a fixed amplitude and one of four possible phases, a configuration often described as four-point quadrature-amplitude modulation (4-QAM). Two bits can be used to define which of the four phases is used on each carrier. The REVERB signal is formed by encoding two bits onto each carrier, beginning with carrier #0 and continuing through carrier #255. In practice some carriers may be missing, and one carrier is appropriated for use as a pilot tone.

0025The bits to be encoded on each carrier of the REVERB signal are generated by a pseudo-random bit sequence (PRBS) generator defined by the polynomial <br />1+x⁻⁴+x⁻⁹  (1)

0026<figref idref="DRAWINGS">FIG. 3</figref> is a schematic representation of the above polynomial, and is one specific embodiment of the PRBS generator of <figref idref="DRAWINGS">FIG. 2</figref>. PRBS generator <b>301</b> is defined by (1), a polynomial of degree 9, and has a period of 2⁹−1=511. As such, the generator can produce 511 distinct pseudo-random sequences of 511, each sequence being a shifted version of any other sequence. ITU recommendation G.992.1 defines which of those 511 sequences is to be used to form the REVERB signal by specifying the first nine outputs of the PRBS to be all ones. Specifically, G.992.1 defines the PRBS sequence {d_n} as follows: <br /><i>d</i>_n=1 for <i>n=</i>1, 2, . . . , 9  (2a)<br /><i>d</i>_n<i>=d</i>_n−4<i>⊕d</i>_n−9otherwise  (2b)<br /> where the symbol ⊕ is used to denote the exclusive OR (XOR) operation.

0027This definition is equivalent to specifying the initial state, {i₁, i₂, . . . i_N}, of the shift register in <figref idref="DRAWINGS">FIG. 3</figref> to be, {0,0,0,0,0,1,1,1,1}, abreviated here as 000001111. This initial condition assumes that bits are read from left to right (most significant bits are 00000, least significant bits are 1111) and that the first output of the circuit is computed before the first shift is executed. The total number of bits required to specify the encoding for all 256 carriers in a REVERB symbol is 512 (256 carriers, two bits per carrier). If the rule in (2) is used to calculate 512 outputs, the 512^thoutput will be the same as the first output.

0028In accordance with the system and method of the present invention, the principle outlined in (2) is used to generate a signal or signals other than the REVERB signal. For a given set of active carriers, exactly 511 such signals may be defined, one for each cyclic shift of the basic PRBS sequence defined by (2) (<figref idref="DRAWINGS">FIG. 3</figref>). The length of the sequence defined by (1) is one less than the length of the sequence needed to produce 256 pairs of bits to encode an entire REVERB symbol. That is, if the first symbol is defined by generating 512 PRBS outputs (after initializing the PRBS shift register according to rule (2a)), then a new sequence of 512 outputs is computed as follows. First, the PRBS shift register is not reinitialized, but the initial state of the generator for the new sequence is taken to be the final state resulting from generation of the previous sequence. This procedure produces a PRBS sequence different from the first. If the process is repeated 511 times, exactly 511 distinct PRBS sequences are generated.

0029Next, each of the 511 distinct sequences generated (as set forth above) is used to generate an ADSL symbol, the PAR of each of which is then computed/determined. Finally, the signal with the lowest PAR is selected, and the corresponding initial value of the PRBS generator is noted. Accordingly, the signal with the lowest PAR is determined from the 511 signals considered. Additional detail regarding the determination of the signal having the lowest PAR is discussed below with respect to <figref idref="DRAWINGS">FIGS. 5 and 6</figref>.

0030In another (or complimentary) embodiment, different gains may be assigned to each of the carriers that forms a symbol. In the Q-mode application, this gain would be the same as that employed on the same carrier during SHOWTIME.

0031<figref idref="DRAWINGS">FIG. 4</figref> is a functional block diagram of one embodiment of a method for computing/determining the PAR of an ADSL symbol. The embodiment of <figref idref="DRAWINGS">FIG. 4</figref> starts with a frequency-domain description of the symbol consisting of a set of complex amplitudes, {X_r; r=0, 1, . . . , N−1} (reference numeral <b>401</b>). Next, at block <b>403</b>, the real inverse discrete Fourier transform (RIDFT) of {X_r} is computed yielding {x_k; k=0, 1, . . . , 2N−1}, a block of 2N real time domain samples (reference numeral <b>405</b>). The {x_k} sequence is then interpolated (block <b>407</b>) to form {x′_i; i=0, 1, . . . , 2NM−1} where <br />x′_Mi=x_i.

0032{x′_i} is a block of 2NM real time domain samples (reference numeral <b>409</b>). The {x′_i} sequence is next filtered with {h_i; i=0, 1, . . . , L−1} (block <b>411</b>), a filter representing the A-to-D converter, analog filter, and line driver of the transmitter. The output of the filter is the sequence {y_i; i=0, 1, . . . , 2 MN+L−2} where <maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mrow><msub><mi>y</mi><mi>i</mi></msub><mo>=</mo><mrow><munderover><mo>∑</mo><mrow><mi>j</mi><mo>=</mo><mn>0</mn></mrow><mrow><mi>L</mi><mo>-</mo><mn>1</mn></mrow></munderover><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mrow><msub><mi>h</mi><mi>j</mi></msub><mo>⁢</mo><mrow><msubsup><mi>x</mi><mrow><mi>i</mi><mo>-</mo><mi>j</mi></mrow><mi>′</mi></msubsup><mo>.</mo></mrow></mrow></mrow></mrow></math></maths>

0033One symbol's worth (i.e., 2NM) of time samples is collected (block <b>413</b>). Next, the maximum value of the absolute value of the elements of {y_i}, Y_max, and the value of the RMS value of {y_i}, Y_RMS, are found (block <b>415</b>) where <maths id="MATH-US-00002" num="00002"><math overflow="scroll"><mrow><msub><mi>y</mi><mrow><mi>R</mi><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>MS</mi></mrow></msub><mo>=</mo><mrow><msqrt><mfrac><mrow><munderover><mo>∑</mo><mrow><mi>i</mi><mo>=</mo><mn>0</mn></mrow><mrow><mrow><mn>2</mn><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>MN</mi></mrow><mo>-</mo><mn>1</mn></mrow></munderover><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><msubsup><mi>y</mi><mi>i</mi><mn>2</mn></msubsup></mrow><mrow><mn>2</mn><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>MN</mi></mrow></mfrac></msqrt><mo>.</mo></mrow></mrow></math></maths>

0034Finally, the PAR for the ADSL symbol under consideration is computed as <maths id="MATH-US-00003" num="00003"><math overflow="scroll"><mrow><mrow><mi>PAR</mi><mo>=</mo><mfrac><msub><mi>y</mi><mi>max</mi></msub><msub><mi>y</mi><mrow><mi>R</mi><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>MS</mi></mrow></msub></mfrac></mrow><mo>,</mo></mrow></math></maths><br /> (block <b>417</b>) a ratio often expressed in dB as <br />PAR_dB=20 log₁₀(PAR).<br /> This process is repeated for each ADSL symbol generated, as mentioned above.

0035<figref idref="DRAWINGS">FIG. 5</figref> is a flow diagram of one embodiment for obtaining a signal with the lowest PAR using a particular PRBS generator. First, the initial state of the PRBS generator is selected (block <b>501</b>). Next, an output from the PRBS generator is obtained (block <b>503</b>). The PRBS generator used and its initial state may be those discussed above with reference to <figref idref="DRAWINGS">FIG. 3</figref>, for example. The PAR of the symbol formed is then calculated by modulating an ADSL symbol with the PRBS output (block <b>505</b>). The PAR may be calculated using the method discussed above with reference to <figref idref="DRAWINGS">FIG. 4</figref>. Next, the calculated PAR and the corresponding initial state are stored in a table, for example (block <b>507</b>).

0036A determination is then made whether the system is “done” (block <b>509</b>). “Done” may mean that either the system has performed all possible calculations based on the PRBS generator length, or has performed enough calculations to obtain a reasonable representative sample (e.g., 1000). If the system is not “done,” the process is repeated (starting at block <b>503</b>). As mentioned above, the PRBS generator is not reinitialized each time the process of <figref idref="DRAWINGS">FIG. 5</figref> is repeated. Instead, the initial state of the PRBS generator for each new iteration is set as the final state of the PRBS generator that results from the previous iteration.

0037Finally, when the determination is made that the system is indeed “done” (block <b>509</b>), the minimum PAR value is selected from the table (block <b>511</b>). Also, the corresponding initial state of the PRBS generator is noted (block <b>511</b>).

0038<figref idref="DRAWINGS">FIG. 6</figref> is a more detailed flow diagram of one embodiment of a method for obtaining a signal with the lowest PAR. First, the system is initialized, with the counter or iteration variable, i, being set to 0, the PRBS state variable, s, being set to s₀, and minPAR being set to a large number (block <b>601</b>). The initial PRBS state is then selected to be s₀(block <b>603</b>) where s₀does not correspond to the all-zero state. Next, a sequence of outputs from the PRBS generator is obtained (block <b>605</b>) sufficient to modulate one ADSL symbol. The PRBS generator used and its initial state may again be those discussed above with reference to <figref idref="DRAWINGS">FIG. 3</figref>, for example. The PAR of the symbol formed is then calculated by modulating an ADSL symbol with the PRBS output (block <b>607</b>). The PAR may again be calculated using the method discussed above with reference to <figref idref="DRAWINGS">FIG. 4</figref>.

0039Next, a determination is made whether the PAR calculated is less than minPAR (block <b>609</b>). If the calculated PAR is indeed less than minPAR, then minPAR is reset as PAR, and the corresponding initial state of the PRBS generator (i.e. s_min, the initial state corresponding to minPAR) is set to be s_i. If the calculated PAR is not less than minPAR, then the previous minPAR and s_minare maintained.

0040In either case, the counter or iteration variable, i, is incremented (block <b>613</b>), and a determination is made whether i is greater than or equal to i_max(block <b>615</b>). In other words, a determination is made whether all iterations have been completed (i.e., whether the system is “done”). If all iterations have not been completed (i.e., i is less than i_max), then, another iteration is undertaken (starting at block <b>605</b>).

0041Again, as mentioned above, the PRBS generator is not reinitialized for each iteration. Instead, the initial state of the PRBS generator for each new iteration is set as the final state of the PRBS generator that results from the previous iteration. In other words, the PRBS state at the start of each iteration (i.e., immediately before the output sequence from the PRBS is generated at block <b>605</b>) is s_i.

0042Finally, when the determination is made that i is indeed greater than or equal to i_max, minPAR and the corresponding s_minare output (e.g., selected from memory) (block <b>617</b>).

0043Based on the above, the overall operation of one embodiment of the system based on the PRBS defined by (1) may take place as follows:

00441. Characterize the A-to-D converter/filter/line driver of <figref idref="DRAWINGS">FIG. 4</figref> for the particular implementation of the ADSL modem per the example discussed above. (i.e., determine {h_i; i=0, 1, . . . , N−1}.

00452. During modem training note the set of active carriers to be used.

00463. Using that set of carriers, <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0000"><ul id="ul0002" list-style="none"><li id="ul0002-0001" num="0047">3.1 Initialize the shift register of the PRBS defined in (1) and in <figref idref="DRAWINGS">FIG. 3</figref> to any non-zero value.</li><li id="ul0002-0002" num="0048">3.2 Operate the shift register 512 times, grouping the output bits into groups of 2.</li><li id="ul0002-0003" num="0049">3.3 Form a frequency-domain ADSL symbol by modulating the i^thactive carrier with the i^thpair of bits from the set obtained in Step 3.2 for i=0 to 255 using any subset of the entire set of carriers and any desired set of gains for the set of active carriers.</li><li id="ul0002-0004" num="0050">3.4 Calculate the PAR of the resulting symbol using the method discussed with respect to <figref idref="DRAWINGS">FIG. 4</figref>.</li><li id="ul0002-0005" num="0051">3.5 Repeat the above steps 2⁹−2=510 times starting with Step 3.2, remembering at each step the minimum PAR obtained as well as the initial state of the PRBS shift register that produced that minimum.</li><li id="ul0002-0006" num="0052">3.6 The result is the overall minimum PAR as well as the initial state of the PRBS shift register that produced that overall minimum.</li></ul></li></ul>

0053In the greater scheme of Q-mode operation, it may be desirable for one modem to calculate the best (i.e., lowest PAR) Q-mode signal and to communicate the results of its calculations to the remote modem. A specification for the Q-mode signal calculated by the transmitter on one end of a connection may need to be communicated to the receiver on the opposite end of the connection. Transmitting only the bits that define the initial state of the PRBS generator is sufficient to inform the remote receiver how to construct the Q-mode signal.

0054<figref idref="DRAWINGS">FIG. 7</figref> is a flow diagram of communication between ADSL modems according to one embodiment of the present invention. First, a PRBS generator is selected/agreed on by the modem(s) (block <b>701</b>). Next, minPAR and s_minare determined (block <b>703</b>). This may be achieved by the method discussed above with respect to <figref idref="DRAWINGS">FIG. 6</figref>. The s_mindetermined is then transmitted to the remote modem (block <b>705</b>), and received by the remote modem (block <b>707</b>). Finally, the remote modem uses the s_minreceived to determine the Q-mode signal to be used (block <b>709</b>).

0055While one embodiment of the invention discussed above assumes that the polynomial of (1) (see <figref idref="DRAWINGS">FIG. 2</figref>) defines the PRBS generator to be used to define the ADSL Q-mode symbol, other PRBS generators may also be used. For example, the shorter PRBS defined by <br />1+x⁻²+x⁻³+x⁻⁴+x⁻⁸  (3)<br /> can be used as well. In fact, in one study of 26 cases, this shorter PRBS (its length is 2⁸−1=255) leads to a smaller minimum PAR than did (2) on 24 of the 26 cases. Therefore, there is no need to restrict the length of PRBS to be used to generate the low PAR signals. Exactly the same procedure described above may be used for any PRBS.

0056Many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as described hereinabove.